The present invention relates to a touch panel device for detecting the contact of an object, such as a finger or a pen, with the touch panel device, and more particularly relates to a touch panel device and a contact position detection method, for detecting the contact position of the object by detecting attenuation and cutoff of surface acoustic waves (SAWs).
With the spread of computer systems, mainly personal computers, there has been used a device for inputting new information or giving various instructions to a computer system by pointing at a position on the display screen of a display device on which information is displayed by the computer system, with a finger or a pen. In order to perform an input operation with respect to the information displayed on the display screen of the display device of a personal computer or the like by a touching method, it is necessary to detect the contact position (pointed position) on the display screen with high accuracy.
Well known examples of a touch panel device for detecting the contact position of an object such as a finger or a pen are a device using a resistance film, and a device using ultrasonic waves. The former device using a resistance film detects a change in the resistance of the resistance film caused by contact of the object with the resistance film. This device has the advantage of low consumption of power, but has problems in the aspects of the response time, detection performance and durability.
By contrast, in the device using ultrasonic waves, the contact position of an object such as a finger or a pen is detected by propagating surface acoustic waves on a non-piezoelectric substrate, for example, and detecting attenuation of the surface acoustic waves caused by contact of the object with the non-piezoelectric substrate. In general, this touch panel device has a structure in which a burst wave is applied to cause a transducer to generate surface acoustic waves, the generated surface acoustic waves are propagated on the non-piezoelectric substrate, the propagated surface acoustic waves are received, and the contact position of the object is obtained based on the received results. A variety of such touch panel devices have been proposed (for example, Japanese Patent Application Laid-Open No. 7-319613/1995).
The present inventor et al. is conducting research and development on a touch panel device that uses, as a transducer, an IDT (inter digital transducer: comb-like electrode) that can be produced collectively using a photolithography technique. In this touch panel device, an element composed of an IDT and a piezoelectric thin film is used as each of excitation elements for exciting surface acoustic waves and receiving elements for receiving propagated surface acoustic waves.
FIG. 1 is an illustration showing the configuration of such a touch panel device using IDTs. In FIG. 1, the numeral 61 represents a rectangular non-piezoelectric substrate. A plurality of excitation elements 62, each composed of an input IDT and a piezoelectric thin film, for exciting surface acoustic waves are arranged in a line on one end of each of the X-direction and the Y-direction of the non-piezoelectric substrate 61 so that the excitation elements 62 correspond to a plurality of tracks, respectively. Moreover, a plurality of receiving elements 63, each composed of an output IDT and a piezoelectric thin film, for receiving surface acoustic waves are arranged in a line on the other end of each of the X-direction and the Y-direction of the non-piezoelectric substrate 61 so that the receiving elements 63 face the excitation elements 62.
In this touch panel device, a burst wave is applied to each of the excitation elements 62 to excite surface acoustic waves and propagate them on the non-piezoelectric substrate 61, and then the propagated surface acoustic waves are received by the receiving elements 63. When an object such as a finger or a pen is in contact with the propagation path of a surface acoustic wave on the non-piezoelectric substrate 61, the surface acoustic wave attenuates. Accordingly, by detecting whether or not the level of the received signals at the receiving elements 63 is attenuated, it is possible to detect the presence or absence of contact of the object and the contact position.
In addition, the present inventor et al. proposed a touch panel device in which the excitation elements and the receiving elements are arranged so as to propagate surface acoustic waves in oblique directions (diagonal directions) of the substrate. FIG. 2 is an illustration showing an example of the electrode structure of such a touch panel device. In FIG. 2, the numeral 70 represents a rectangular non-piezoelectric substrate made of glass material, and a center portion enclosed by the broken line is a detection region 70a capable of detecting the contact position.
In a frame region outside the detection region 70a, which is a peripheral section of the non-piezoelectric substrate 70, four IDTs 71 are disposed. Each IDT 71 comprises facing bus electrodes 72, and comb-like electrode fingers 73 which are extended from the bus electrodes 72 by turns and bent in the middle. In this structure, lines of comb-like electrode fingers 73 tilted in two directions from the facing direction of the bus electrodes 72 are formed, thereby realizing excitation of surface acoustic waves in two directions and reception of surface acoustic waves from two directions. In this example, the IDTs 71 on the upper side and the lower side function as excitation elements for simultaneously exciting surface acoustic waves in two different directions, while the IDTs 71 on the left side and the right side function as receiving elements for simultaneously receiving surface acoustic waves from two different directions.
In the touch panel device having the structure shown in FIG. 1 or FIG. 2, two pairs or four pairs of electrode groups are used, and the contact position and/or the contact width of the object are calculated from time-series changes in the received strength of surface acoustic waves (the time domain waveform). The S/N of the received signal is proportional to the number of pairs of electrodes included in the aperture of the receiving element that receives the surface acoustic wave. Therefore, since the number of times of driving of the surface acoustic wave (the wave number of the burst wave applied) for maximizing the strength of surface acoustic waves to be received is determined by the electrode structure, a fixed number of burst wave, which is determined by a design value or determined at the time of activation, is applied to the excitation elements. Besides, a fixed threshold value determined at the time of design or activation is used when calculating the contact position and the contact width.
However, since there is a difference in the performance between the respective electrode pairs, even when the burst wave of the same wave number is applied to propagate the surface acoustic waves, there is a variation in the received strength of the time domain waveform. Thus, when calculating the contact position and the contact width using the fixed threshold value determined at the time of design or activation, differences occur in touch sensitivity on the panel. Moreover, when the panel is dirty with finger prints, etc., the received strength of the time domain waveform varies. As a result, when the panel gets more dirty, the touch sensitivity gradually decreases, and there arises the problem that high pressure of object (tool force when a pen is used) is necessary to calculate the contact position and the contact width. Further, although at least two channels of receiving signals are necessary to calculate the contact position and the contact width, there is the case where the necessary pressure of the object for calculation differs between the respective channels, and only one channel is valid. In such a case, there is the problem that the contact position and contact width of the object cannot be correctly calculated. Therefore, the present inventor el al. continues to conduct further research and development to solve these problems.